(Andy Balaam, November 2012)
After a significant amount of soul-searching, I chose the MIT Licence for Pepper.
My default choice for other projects has been GPLv2 - I like its goal of creating a Free world of software, with participants who are committed to reciprocating for the benefits they gain.
However, a programming language and library is very different from a "normal" project - its potential users are software producers, not just end users. Making Pepper’s library GPL would prevent anyone but GPL developers using it (even people working on Free software with another licence). Any restriction on use at all is a huge decision, and may prevent someone using it (either for good reasons, or simply out of confusion or fear).
What’s important here is that Pepper is intended to be incorporated into other projects, at least at the level of its library, and also as a language interpreter and compiler. That means it needs to be compatible with everyone else’s license, so developers releasing code under GPL, Apache, BSD, MIT, MPL, other OSS, and proprietary licenses could all incorporate Pepper’s code into their own.
I want everyone, including proprietary developers, to be able to use Pepper for anything, and I want it to be absolutely clear to everyone, without the need for lawyers, that their use case is allowed.
The only restriction I wanted to add was that I didn’t want people to be able to stop others using Pepper. This led me to consider licences that explicitly attempt to prevent patent misuse.
The need for compatibility with lots of other licenses rules out the GPL and other CopyLeft licences, which require redistribution under the same license. The alternative to CopyLeft is permissive. The permissive licences I considered are below.
The Apache Software License 2.0 (ASL2) is a permissive licence, recognised by the Free Software Foundation (FSF), the Open Source Institute (OSI) and Debian Free Software Guidelines (DFSG). It includes a patent clause that people much cleverer than me consider appropriate for avoiding some of the worst evils of software patents. However, because of this patent clause, it is incompatible with the GPLv2, meaning people writing GPLv2 code can’t incorporate code under ASL2.
The Python Software Foundation License is a short, permissive licence recognised by FSF and OSI, and using it would fulfill Pepper’s guiding principle of "when in doubt, make it work like Python". However, it is specific to Python, so would need to be modified for Pepper, and it appears to be near-identical to the MIT licence, which does not require modification in the same way.
The Creative Commons CC0 copyright waiver/licence is a way of giving up your copyright rights, with a backup permissive licence if the waiver is not legal in the relevant jurisdiction. It contains an explicit clause saying it does not have any effect on patents at all. The total freedom offered by giving up all rights is attractive given the goal of anyone being able to use the code, but the explicit allowing of patents seems to me to be a bad thing. The OSI has not accepted that CC0 is an open source licence, because of the patent clause.
The MIT Licence is a very popular and venerable permissive licence, recognised by FSF, OSI and DFSG. It is very short, and places no restriction on distribution except that the copyright and licence notice be preserved.
I like the idea of preventing people from restricting the freedom I am trying to provide using patents, so ASL2 looked good, but GPLv2 code is not going away - there is lots of it, and some people believe it is a better licence than GPLv3, so it will probably never stop being used.
I considered the possibility of dual-licensing Pepper under both ASL2 and GPLv2. This solves the problem of GPLv2 code being able to use Pepper, but introduces lots of complexity (which I don’t fully understand) and might even artificially force a fork some day because different branches needed to work with different licences.
Some people believe that permissive licences without explicit patent exceptions (such as MIT) will be effective in preventing some kinds of patent misuse in some jurisdictions even if they don’t mention patents. This is believed to be true in the UK, for example.
Given the choice of ASL2+GPLv2 which I didn’t fully understand, and the 168 words of the MIT Licence, pretty much all of which I did understand, I went with MIT.
The MIT Licence is one of the oldest open source licences. It is remarkably short, has stood the test of time and clearly intends to allow anyone to use the code for anything.
So, if you want Pepper:
Copyright (C) 2010-2013 The Pepper Developers Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.